There will be a Ten Grands Concert this Friday at Beneroya Hall in Downtown Seattle. As the name suggests, there will be ten pianists each playing a grand piano, all on the stage. They'll be playing a variety of music, both classical and modern. There will also be individual performances, but people definitely go to see all those pianos being played at the same time.
This particular concert is one of a series of fundraising events for the Snowman Foundation, a philanthropic organization which supports music in schools in the Oregon and Southwest Washington areas. The Seattle Times had an article today which covered some of the background of the Ten Grands Concerts and the Snowman Foundation.
If you'd like to see something similar, there's a DVD of a show which aired on PBS called the All-Star Piano Extravaganza: The Verbier Festival and Academy Concert. There are eight pianos in that concert and some well-known pianists, such as Emanuel Ax, James Levine, and Lang Lang. It does seem like they're having fun during the performance.
When I was taking piano lessons, my teacher Mr. Voorhies participated in a monster concert. I don't recall the details, but I believe there were eight pianos and up to three pianists at each. Mr. Voorhies asked me to turn pages for him, so I got to be on stage during the event. The only other pianists I remember were Efrem Zimbalist, Jr. (the actor who was on "The F.B.I."; his father was concert violinist Efrem Zimbalist, Sr. and his mother opera singer Alma Gluck) and Paul Conrad (who does political cartoons).
The most stirring number from that concert was the William Tell Overture, which was definitely a crowd pleaser.
Paul Conrad drew up a cartoon commemorating the event, showing an octopus with each leg playing a piano. Mr. Voorhies gave his to me, and it hung on my parents' wall for quite a while.